How should I save data while is uploading some data with NSURLSession and the app is suspended?

I am developing an app where I have to upload an entity with its relation. (I think this is similar to dropbox files uploading) This entity is called PlaceEntity and has a relation with PictureEntity (to many). I want to do it using NSURLSession to handle the upload while the app is suspended. So, what I am doing is I create the uploadTasks for place pictures, and I start them. After each picture upload I have to receive data and save some information in the pictureEntity. When all the pictures are uploaded I create a new task and I save the place in the server.

I have tested it in background and foreground and everything works fine. The problem is when I suspend the app. The upload picture requests are executed(I know because I watch the request with Charles MAC OS app) the problem happens when I have to update the information in core data, after each upload. The method:

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data

never is called because my app is completely out. I can't then update Core Data or launch a new request.

Any idea??

Thank you